A wind power plant (also called a wind farm) comprises a number of wind turbines: Each wind turbine includes a rotor that converts the kinetic energy in the wind into mechanical energy. The mechanical energy is then converted to electricity by a generator which is also included in the turbine.
When the wind power plant is connected to a power grid, the electricity generated by the generators is delivered to the power grid, which transmits electricity to different consumers/loads. This connection is done through at least one connection point.
Management of the power grid is a difficult task in order to provide customers with a certain level of electricity quality. It is desirable that the power grid has well defined and constant parameters, such as a certain nominal grid voltage and a nominal frequency, e.g. 50 Hz in Europe and 60 Hz in the US.
The power grid links different electrical installations, i.e. loads or generators, such as power plants, each of them having a behaviour as seen from the grid. For example, some consumers/loads behave like impedances and some generation power plants use synchronous generators with different turbines and controllers.
However, the stability of the different grid parameters is dependent on a variety of variables including the balance between generated power and consumed power in the grid in real time. Any imbalance leads to changes in e.g. the grid frequency. When more power is generated than consumed in the grid, the frequency tends to increase. When more power is consumed than generated, the frequency tends to decrease.
A problem with wind power plants is that they generally work in an unpredicted way, from a power grid point of view, due to variations in e.g. the wind speed. As a result, it is hard to ensure that the wind power plant will interact optimally with the power grid.
As the proportion of electrical energy produced by wind power plants increases compared to the total electrical energy produced from conventional power plants in a power grid, the power grid is more and more influenced by the way the wind power plants work. Therefore, it becomes more and more important for wind power plants in contributing to help maintain the characteristics of the power grid by controlling e.g. the real power output of a wind power plant. This would enable a more efficient utilization of all production units, and would improve the possibilities for power grid planning.
EP1467463 describes a wind power plant and a method for operating the wind power plant, which method includes controlling the real power output of the wind power plant depending on a measured frequency in the power grid to which the wind power plant is connected. The output of real power of the wind power plant is regulated or controlled according to the measured grid frequency.
WO2006120033 discloses a wind power plant and a method for operating it, which method includes controlling the power factor or the reactive power of the wind power plant depending on a measured voltage in the power grid to which the wind power plant is connected. In the invention according to WO2006120033, the real power output or the reactive power of the wind power plant is regulated or controlled according to the measured grid voltage.
Thus, both EP1467463 and WO2006120033 describe control methods that depend on only one grid parameter (frequency and voltage respectively). In each of the methods, the choice of parameter is predetermined and can not be changed. Furthermore, the control method is fixed and is always performed in the same way.
However, there is a desire for wind power plants that are controlled in a more flexible way, such that the way the wind power plants behave may vary in time. There is also a need for a method wherein the choice of parameter to be used for controlling the wind power plant can vary in time.